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Old 26th May 2010, 13:37
  #1184 (permalink)  
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I was looking through the old AF447 thread for information and found a relevant post by Greybeard (#3399) which I will quote.
Pitot Ice

Pitot probes have a liquid drain (bleed) hole, whose air bypass has to be accounted for in the airspeed calcuation. Clog the drain, and pitot pressure rises, giving erroneous high airspeed. Indeed, per reading in this thread, some of the earlier Airbus pitot malfunctions were with poorly manufactured drain (bleed) holes.

Add more ice, and you clog the pitot head itself, and then pitot pressure will lock, or will decrease if there is any drain opening at all.

Prior incidents of A330 pitot icing have resulted in overspeed warnings as well as low airspeed warnings.

But look at this post by Jeff (Hyperveloce) on the 9th of July!! #3397.
Out of the loop ?

A contribution by Pilotaydin, on the Airliners.net forum (Pitot freezing=>overestimated airspeeds=>nose up=>stall):
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I would like to share a small story about something i experienced in the sim a while back, as a demo from my instructor towards the pros and cons of fbw and envelope protection...

we "flew" through an area of icing in the sim, the probe heat function and the airbus a/c itself is designed to fly through known icing, however, that doesn't mean it can withstand anything put in its path... our pitots iced over and our airspeeds started indicating 300+ at high altitude, which is bad news, because we're passing Mmo and Vmo, so the a/c as per design pitched up.... after about 20 seconds of this, as the speed wasn't decreasing, we were actually stalling and losing altitude, and the sidestick = useless, it wouldnt let any one of us pitch down, we started a large rate of altitude loss. Even if we disconnected the a/thr system and idled or added full power, the damn nose was pitched up....we went down 30,000 feet into the water outside jfk in the sim....during that descent, nothing came up on the ecam, just the warning chimes of overspeed.......we of course didnt just sit there, it was a demo we were observing he different things going on...at one point my hand did go up towards the PRIM 1 and PRIM 2 computers...i thought maybe if i let them out of the loop, we could go to altn law but i decided not to intervene to see the outcome....
knowing your systems helps, and the a/c doesnt always provide an answer to us....

Things need interpretation and over automation sometimes leaves us out of the loop...the other day over the atlantic, at 35,000 feet, we got master caution chime that said :
Start valve open and it asked us to switch off the bleed to one engine....leaving us with only one bleed left over the atlantic....are you gonna follow the ecam? or are you going to emergency cancel it?
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btw, I don't get how a Pitot obstruction only (of the ram port/the drain) can lead to overestimated airspeeds (and possible overspeed alarms): if the dynamic pressure cannot be overestimated (?), then it has to be the static pressure which is underestimated (?) and this would imply that the static ports are also blocked and that altitude has been lost since their blockage ? Is that so ? Would there be other ways to overestimate airspeeds ? On the contrary, a blocked Pitot ram port is sufficient to roll back the airspeed to 70-80 kts.
Makes the hair on the back of my neck stand up!
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